I don't think they'll find a full time replacement for him in the end. To replace such a player you have to completely change the squad, which PSG categorically do not want to do. They do not want to change their transfer policy and this is their main problem, they think you can buy a player for a lot of money and only from that he will play better, but no.
With time running out in this season's transfer saga it looks like PSG will be starting with something new and I think that's well thought out by Enrique.
The plan to bring in Osimhen and Kvaratskhelia is also likely to fail this time due to the difficulty of negotiating with Napoli about the large release clause but indeed this could be a re-start for PSG so that they know what to do when there is no Mbappe in their squad as well as to see which attacking gaps are weak when they do not have a player of Mbappe's caliber who always made hopes for PSG last season.